There are 2 options you can use to access and manage vCenter Server 5.5. first option is Web client and the second option is still vSphere Client. There is one version vSphere Client will be alive and then it will be history. Vmware wants to use only Web client in the future. However, I remember that there were many decision taken before and most them had to postpone. We will see how series they are

After you installed the Client, you can connect to the vCenter. However, the problem is just starting. As I mentioned before, Vmware is trying to decommission vSphere Client. Therefore, you can access the new features of Vmware 5.5 via vSphere Client